3D Microwave Print Head Approach for Processing Lunar and Mars Regolith

Introduction: NASA is evaluating various methods to process lunar and Mars regolith in preparation for future missions. We have performed earlier studies [1-3] to understand the mechanism associated with the lunar regolith’s excellent absorbtion of microwave energy that is useful for microwave processing. Using an upgraded microwave heating facility we have shown that the previously observed [4] enhanced heating is a consequence of the microwave volumetric heating of the samples. This enhanced heating effect was observed in both highland and mare lunar simulants. We have also demonstrated that this enhance heating effect occurs in Mars regolith simulants. Volumetric heating leads to more efficient heating of the interior of the sample compared to the sample surface. This situation arises because the sample surface radiates energy while the heat produced in the interior only slowly dissipates due to thermal conduction to the surface. This effect can produce a significant temperature gradient leading to sintering or melting in the sample interior. Now that we understand how to control the cause of the sintering and melting we plan to proceed to develop a 3D microwave print head facility. Microwave Measurements and Analyses: We performed a microwave heating study to determine the optimum heating parameters and to understand the internal melting process. All studied samples were initially outgassed at 200oC for 3 hours and the sample density was determined before and after being heated. A 200-Watt TWT microwave amplifier was used to excite our waveguide cavity in a TE104 mode at 2.45 GHz to heat a sample. The sample to be processed was placed in a quartz holder positioned within the waveguide cavity at an electric field maximum. There are two distinguishing feature of this heating facility; a frequency tracker continually tracks the resonant frequency during heating to maximize the electric field strength in the cavity, and an impedance tracker continually adjusts the coupling between the power source and cavity to maintain critical (maximum) coupling into the cavity. We initially addressed the concern that the enhanced heating effect could be associated with a chemical interaction with oxygen in the surrounding atmosphere. To test this concern we repeated our heating measurements on a lunar JSC-2A simulant with a nitrogen atmosphere surrounding the cavity. The resultant heating profile was essentially the same as when heating in earth’s atmosphere. The remaining studies were then performed using earth’s atmosphere. Our microwave heating facility was controlled by a LabVIEW program. With this system we were able to quickly heat a Mars simulant sample to a surface temperature of 435oC as shown in Fig. 1. This figure also shows the cooling rate when the microwave power was turned off. When lunar or Mars samples were heated to a higher (surface) temperature of ~700oC they would completely melt as shown in Fig. 2. Chemical analyses and optical microscopy were performed on various JSC-2A samples heated in our microwave facility. Elemental analysis was performed using a Horiba Model XGT-5000 X-Ray Fluorescence Microscope (μXRF). Powdered samples (some before heating and others after heating) were analyzed using Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy. MicroRaman spectroscopy was performed with a Bruker Senterra system equipped with a 532nm laser. In addition, Thermal Analysis using a TA Instruments Q Series Differential Scanning Calorimeter (DSC) and Thermogravimetric Analysis (TGA) system 400